The present invention relates to an improved device for DCT encoding, quantization and variable word length encoding of digital video signals with a reduction of redundancy of the same video signals.
As it is known, into the DCT encoding (Discrete Cosine Transform) of digital video signals, the video data of each frame or field of the video signal are divided in blocks of, for example, 8.times.8 pel, and the data of each block are treated with a conversion algorithm (Discrete Cosine Transform) that reduces the correlation inside each block. In the more advanced types, the conversion algorithm, in order to reduce the global correlation even more, takes into account one or more preceding frames or fields to draw a prediction. In any case the transformed data are subsequently quantized, scanned according a preset scanning track, and encoded with variable-length words according to an assignment table in which shorter codes are assigned to more frequently occurring data and, since uninterrupted sequences of zeroes are very frequent, specific codes are assigned for zero sequences of different lengths. For a more detailed description of these techniques see, for example, Rafael C. Gonzales and Paul Wintz, "Digital Signal Processing", 2nd edition, Addison Wesley, pages 288-289.
As it is known, the essential aim of the above-cited techniques is that of reducing the mean volume of digital video signals to be transmitted in a transmission channel, for the same definition of the transmitted image. However, such signal processing techniques yield a time-variable data flow, namely greater for complex or rapidly moving images and lower for static and/or scarcely detailed images. In order to prevent the occasional saturation of the channel because of that, the quantization resolution is reduced as the generated data flow grows.